Snape

The way you have posed Snape makes for a dramatic and dynamic portrait; the way his left hand reaches out to the viewer draws us in and involves the viewer in the action - almost as though he might be casting his spell upon us. I think this is a major strength of the composition. The one uncomfortable element of the composition for me is the smallness of his right hand (the one holding the wand) it needs to be larger - you could test this out by positioning your own hand in front of the mirror and checking out it's size when compared to the size of your face,
I like the dark treatment of the portrait and how this contrasts to the brightness of his eyes and the highlights upon his hands. It's clear that a lot of thought has gone into the creation of this portrait with the play of dark and light, and the drama of the piece.